An example of prior-art pattern tracing systems of the nature above mentioned is disclosed in U.S. Pat. No. 3,423,589 showing a photoelectric pattern-contour tracing system in which a sensing head having two discrete photocells is rotatable about its center axis and is movable over and along the pattern contour to be traced. The sensing head is positioned with respect to the pattern contour in such a manner that the two photocells are spaced apart in a direction perpendicular in non-intersecting relationship to the pattern contour. When the sensing head assumes a position having the photocells located symmetrically with respect to the width of the pattern contour, viz., when the center line of the spacing between the photocells is located accurately above the longitudinal center line of the pattern contour, the photocells are subjected to light of equal intensities so that the currents respectively produced by the photocells are equal in magnitude to each other. When the sensing head is deviated from the correct position over the pattern contour, the two photocells sense light of different intensities and thus produce currents with different magnitudes. The path along which the sensing head is to travel is adjusted upon detection of such a difference between the magnitudes of the currents respectively produced by the two photocells.
Since the adjustment of the path along which the sensing head is to advance is thus made through detection of the currents actually produced by the photocells moved with the sensing head, the sensing head must be moved and turned in various directions before a correct path is determined. For this reason, the sensing head must be equipped or associated with a disproportionately large number of mechanical members and structures which are subject to erroneous motion and failure during operation and which will add to the measurements, weight and production and mainterance costs of the tracing system as a whole. Because, furthermore, of the fact that the sensing head essentially consists of only two photocells which are juxtaposed in close proximity to each other and which are to be moved along the pattern to be traced, the sensing head can not be satisfactorily sensitive to acute angles and to lines extending in close proximity to each other.
Another example of known pattern tracing systems is taught in Japanese patent Publication No. 49-42803 showing a photoelectric pattern tracing system using a two-dimensional matrix array consisting of a number of photoelectric transducer elements arranged on xy-coordinates. The matrix array is positioned over the pattern to be traced and produces binary signals from those transducer elements which are located above a portion of the pattern. When the matrix array assumes a certain position over the portion of the pattern, one of the transducer elements produces binary signals indicating the location of a specific point of the portion of the pattern in terms of the xy-coordinates of the matrix array and another one of the transducer element produces binary signals indicative of the location of the leading end of the portion of the pattern in terms of the xy-coordinates. The matrix array is thus moved from the position indicated by the binary signals proudced by the former transducer element to the position indicated by the binary signals produced by the latter transducer element. Only the direction in which the portion of the pattern to be traced is detected by the matrix array, which is therefore not responsive to the width of the pattern and which is accordingly not capable of recognizing acute angles and lines which are close to each other.
The present invention contemplates elimination of these and other drawbacks inherent in prior-art pattern tracing systems of the described general natures.